GRID IONIZATION CHAMBER MEASUREMENTS OF ELECTRON DRIFT VELOCITIES IN GAS MIXTURES

Abstract

Electron drift velocities have been measured in the low X/p region from 0.05 to 0.5 volts per cm. per mm. mercury which is an important one for the calculation of collection times in electron pulse chambers. The grid ionization chamber method is rapid and convenient and gives an accuracy of about 10%. Mixtures of the noble gases neon, argon, krypton, and xenon with carbon dioxide and/or methane have been studied, as well as a number of unpurified "tank" gases. In the mixtures used there are anomalies, or humps, in the drift velocity curves which seem to be correlated with the "Ramsauer" electron free paths in the four noble gases. There is a striking difference in the effect of carbon dioxide and of methane when added in small amounts to the noble gases, which is attributed mainly to the difference in electron mean free path in these two gases, but may also be an indication of a difference in electron energy distributions.